The present invention relates to a method of producing cube-on-edge oriented silicon steel strip and sheet for magnetic uses. Cube-on-edge orientation is designated (110) [001] in accordance with the Miller Indices. The method of the present invention has utility for the production of both so-called regular grade and high permeability grade material containing from about 2% to 4% silicon of uniform magnetic properties, from a strand or continuously cast slab of a thickness suitable for direct hot rolling.
As described in U.S. Pat. No. 3,764,406, issued Oct. 9, 1973 to M. F. Littman, cube-on-edge oriented silicon steel strip or sheet is generally made by melting a silicon steel of suitable composition, refining, casting, hot reducing ingots or slabs to hot rolled bands of about 2.5 mm thickness or less, optionally annealing, removing scale, cold reducing in at least one stage to a final thickness of about 0.25 to about 0.35 mm, decarburizing by a continuous anneal in a wet hydrogen atmosphere, coating with an annealing separator and box annealing for several hours in dry hydrogen at a temperature above about 1100.degree. C.
Two conditions must be satisfied before the high temperature portion of the final box anneal during which secondary recrystallization occurs, in order to obtain material having a high degree of cube-on-edge orientation:
(1) A suitable structure of completely recrystallized grains with a sufficient number of these grains having the final cube-on-edge orientation;
(2) The presence of inhibitors in the form of small, uniformly distributed inclusions which restrain primary grain growth in the early portions of the anneal until a vigorous secondary growth occurs during the latter, high temperature portion of the anneal.
During the secondary grain growth portion of the final anneal, the cube-on-edge grains consume other grains in the matrix having a different orientation.
U.S. Pat. No. 2,599,340, issued June 3, 1952 to M. F. Littmann et al, discloses a process for the production of cube-on-edge oriented silicon steel wherein slabs rolled from ingots are heated to a temperature above about 1260.degree. C., and particularly from about 1350.degree. to about 1400.degree. C. prior to hot rolling. This heating step not only prepares the metal for hot rolling but also dissolves the inhibitor present therein so that upon subsequent hot rolling the inhibitor is precipitated in the desired form of small, uniformly distributed inclusions, thereby satisfying one of the two essential conditions for obtaining highly oriented cube-on-edge material. The primary grain growth inhibitor is usually manganese sulfide, but other inhibitors such as manganese selenide, aluminum nitride, or mixtures thereof may be used.
Strand casting into a continuous slab or casting into individual slabs of a thickness suitable for direct hot rolling is advantageous in comparison to ingot casting, in avoiding the loss of material from the butt and top portions of conventional ingots, which ordinarily must be cropped, and in decreasing the extent of hot reduction required to reach hot band thickness. However, when strand cast slabs of silicon steel are produced, a columnar grain structure is obtained which extends from each surface inwardly almost to the center of the slab, with a relatively narrow core or band of equiaxed grains at the center. When such a slab is heated above about 1300.degree. C. prior to hot rolling by the process disclosed in the above U.S. Pat. No. 2,599,340, excessive grain growth occurs. The average diameter of grains after reheating above 1300.degree. C. is about 25 mm (about 0.5-1.0 ASTM grain size at 1x). In comparison, the average grain diameter in slabs rolled from ingots after reheating above about 1300.degree. C., is about 10 mm.
The above-mentioned U.S. Pat. No. 3,764,406 discloses and claims a solution to the problem of excessive grain growth, by heating a cast slab to a temperature of at least about 750.degree. C. but below about 1250.degree. C., initially hot reducing or prerolling the slab with a reduction in thickness of 5% to 50%, followed by the conventional step of reheating the slab to a temperature between about 1260.degree. and 1400.degree. C. before proceeding with conventional hot rolling. This heat treatment and prerolling made possible an average grain diameter of about 7 mm or less after reheating above 1300.degree. C. prior to hot rolling. This in turn had a beneficial effect on the development of cube-on-edge texture in the final product and provided greatly improved uniformity in magnetic properties. Preferably the initial heating of the slab in this patent is at a temperature of about 850.degree. to about 1150.degree. C., and the reduction in thickness is preferably between about 10% and 50%, and more preferably about 25%. Column 7, lines 10-14 indicate that as the percent reduction increases over 25%, the benefit in terms of grain size of the reheated slab gradually diminishes.
U.S. Pat. No. 3,841,924, issued Oct. 15, 1974 to A. Sakakura et al, discloses a process very similar to that of U.S. Pat. No. 3,764,406, with the slab being heated initially to a temperature below 1300.degree. C. and subjected to "break-down rolling" (i.e. prerolling) at a reduction rate between 30 and 70% before the conventional hot rolling step. In the specific example, a slab was initially heated at 1230.degree. C., then subjected to prerolling.
In U.S. Pat. No. 3,841,924, the starting material contains not more than 0.085% carbon, 2.0%-4.0% silicon, 0.010%-0.065% acid-soluble aluminum, and balance iron and unavoidable impurities. The relatively high carbon content in the process of this patent helps to overcome the incomplete recrystallization associated with large grains in cast slabs. At column 3, lines 6-9, it is stated that if the slab heating temperature exceeds 1300.degree. C., the columnar structure grows coarse and no substantial effect can be obtained by the subsequent breaking down treatment. This patent tolerates relatively large average grain diameter after reheating, the requirement being merely that more than 80% of the grains after reheating be less than 25 mm in average grain diameter.
U.S. Pat. No. 4,108,694 discloses electromagnetic stirring of continuously cast silicon steel slabs, which is alleged to prevent excessive grain growth in the central equi-axed zone of the slab after reheating to 1300.degree.-1400.degree. C. before hot rolling. This in turn is stated to result in improved magnetic properties in the final product. Electromagnetic stirring is equivalent in its effect to ultrasonic vibration, inoculation, or casting at a temperature very close to the solidus temperature of the metal.
While U.S. Pat. No. 3,764,406 successfully solved the problem of excessive grain growth after reheating above about 1300.degree. C. prior to hot rolling, the process requires extra equipment for the initial heating within the range of 750.degree. to below about 1250.degree. C. Without such extra equipment, the practice of U.S. Pat. No. 3,764,406 will result in reduced output and increased costs for slab reheating and hot rolling by restricting the furnace capacity available for slab reheating above about 1300.degree. prior to hot rolling.
There is thus still a need for improvement in a process for producing oriented silicon steel strip and sheet from strand cast slabs with conventional equipment which will reduce the load on the roughing mill and permit faster dropout rates in slab reheating prior to hot rolling.